Prior radar receivers having moving target indicator (MTI) capability include some means for suppressing clutter present in the radar return signals so that a target return signal may be distinguished, either visually or automatically, from an obscuring background formed by clutter returns from the sea or ground surface and by system noise. One prior type of MTI receiver suppresses clutter by means that include a signal correlator. The correlator multiplies the radar receiver i.f. signal by the i.f. signal that has been passed through a delay line. The delay line provides a delay time comparable to, or somewhat less than, the duration of the transmitted pulse. The correlator output is filtered in a wide-band, band pass filter and the filtered output is compared with a fixed threshold or a slowly varying threshold in a comparator. The fixed threshold is set at a level estimated to be near the level of the filter output when clutter and noise alone compose the correlator input so that no output is generated by the comparator in the absence of a target signal. If a target signal is present in the correlator input, as well as clutter and noise, the filter output rises above the threshold and an output is generated by the comparator, signifying the presence of a target.
In the absence of a target signal, the clutter-x-delayed clutter product of the correlator contains low frequency components which are intended to be removed by the band pass filter. Since some clutter-x-delayed clutter and clutter-x-noise components pass through the filter, a variable threshold is often used to make adjustments for slow variations in clutter level. However, rapid variations in the clutter level, comparable to the variations associated with a target signal, are not accommodated by a slowly varying threshold, leading to false alarms.
Obviously, the ability of such a prior MTI receiver to detect targets in the presence of clutter and noise is dependent upon the accuracy with which the fixed or slowly varying threshold of the comparator duplicates the level of the filter output when the correlator input is composed solely of clutter and noise. Since the clutter level is subject to uncertain changes according to changes in the wind, sea state or terrain, the fixed or slowly varying threshold is likely to be set at too high a level, causing missed target detections, or too low a level, causing false alarms to be generated.
It is a principle object of the invention to provide an MTI radar target discriminator having improved sub-clutter visibility and immunity to false alarms.
Another object of the invention is to provide a target discriminator for an MTI radar in which the reference threshold is adaptively adjusted to correspond to rapid changes in the clutter level actually present, rather than against a fixed or slowly varying threshold.
It is a further object of the invention to provide a target discriminator for an MTI radar in which a target signal does not contribute to the adaptive reference threshold established from the clutter level actually present.
Other objects and advantages of the invention will become evident as a complete understanding thereof is gained from the following detailed description and the accompanying drawings.